what is a cable modem?

[fivepesos]

i sat down and though about that. take mine for example, its external and a Toshiba branded one. I have to plug t he modem into an ethernet port (or hub) to function.

Here are what i know and what i think about it:

1) It only serves DHCP address to the NIC it was first connected to. If i change NICs or computers I have to reset the cable modem.
2) The device doesn't have its own IP.
3) Network traffice is oblivious to it's presents.
4) Before RoadRunner limited assigned IP's, each internal Machine would recieve an IP if i plugged it directly into a hub.

My friend told me it was a router, im certain hes wrong. Routers are Network layer devices. It has to be a bridge or Datalink layer device. Which means it needs to know my MAC (physical) address which is set by the NIC manufacturer. My assumption would support Point1. However, Point4 means it functions as a media adapter. So this must mean it's a bridge and a media converter? if that's so, even if i wanted to, i couldnt sniff on my local node (my neighborhood). ive never tried but should just for EDUCATIONAL purposes. it would be function identical to a bridge or switch (which is a multiport bridge, generally).

is this a correct assumption?

 

[PC Freak]

Here's what I think.
The word 'modem' is really a acronyem for 'modulator - demodulator'
This means that a modem takes a digital signal from the computer and modulates it into a quasi-analog signal so it can be passed over the telephone lines. Then ,of course, it's demodulated back into a digital signal on the distant end. Now. I'm not sure, but i do not believe a cable "modem" acutally modulates/demodulates a signal. I may be wrong.
Anyone have any ideas on this?

 

[dukenukem]

Using the term "modem" to describe this device is a little misleading. It is indeed a modem; it MOdulates and DEModulates signals. But cable modems are much more complicated than a telephone modem. Cable modems are typically part modem, part tuner, part encryption/decryption device, part bridge, part router, part network interface card, part SNMP agent, part DHCP server, part network address translator (NAT), and part Ethernet hub.

Because of the unique envirnment it is difficult to classify a cable modem as you are wanting to do. It does not operate like a typical tranceiver, hub, switch, router, or server. It combines some functionality of each of these.

 

[Supergax]

You don't have reset the cable modem when you change nics, I have done this many a time.

 

[fivepesos]

supergax, with i Toshiba PCX1000 external modem it is necessary. Well, at least the one given to me by Road Runner, they may be configured differently. this post was pretty informative for me, made me reevaluate a few things (so did network layer switches). ohh well thx for the response

 

[Supergax]

I have the same exact modem, and I have never had to reset it, I guess maybe you are suppossed to or something, but I have the same model and never have reset it, including when I add new computers to the little network in my house here.

Also, I think it isn't oblivious to traffic, from what I've read that 10.xx.xx.xx IP address you get on the first hop of a traceroute is the cable modem.

 

[dukenukem]

If your PC address is in any of the following ranges

10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255

then your IP datagram will have to go through a NAT device to translate your private IP address to a public address.


ARP caches can have a configurable timeout value. It may be that on some cable modem configurations it is set to a max value to keep "forever" while others may have a short timeout. Also there may be a configuration setting that allows the device to learn (the correct functionality) when it "hears" data coming from a known IP address even though the MAC is different than it expects.

I suspect that some cable modems are set to not allow this standard functionality to reduce the trouble caused by incorrect configuration of multiple devices on the LAN side and thus must be reset to relearn the MAC address.

Regardless there are multiple opportunities and I assure you that in some instances resetting the cable modem is necessary after a hardware change.